The prior art generally teaches that the plunger is continuously driven, via a combination of tie rods/connecting rods, by a compact drive unit in a sub-structure of the press.
It is known from doctrinal technical literature that presses with bottom drives are predominantly implemented as presses with a small target force and a high number of strokes and not so much as so-called large presses.
It is further indicated that the tie rods/connecting rods acting laterally on the plunder lead to greater bending stress and to a correspondingly great bending of the plunger, but that the line of action of forces acting off-center on the plunger always lies between the pivot points of the tie rods/connecting rods.
The tie rods/connecting rods are also frequently guided in supporting stands (at least above the sub-structure) which are connected to a cross-beam located above the supporting stands and forming the plunger, thus virtually forming a press frame for forces that may occur and are to be absorbed.
The person skilled in the art aims at designing the press with regard to occurring forces, according to the actions required for processing the work pieces and also the reactions to press shocks or bending, so that a supporting stand construction is chosen in the press frame.
As a result of this technical overview, cost-effective solutions are being sought for designing and operating such presses with bottom drives, also as large presses, without the disadvantages set forth in the individual examples below, such as, for example, as a supporting stand construction.
The analysis of exemplary implementations of presses with a bottom drive, known as individual solutions, shows:
AT 215 257 describes a press where the protruding flywheel requires a lot of enclosed space. Due to the complex lever kinematics, potentially required shock absorption becomes ineffective and could, if required, only be compensated for by high material usage. The inevitable transmission of the off-center forces mentioned above is inefficient due to the flexible reaction of the lever kinematics. The relatively high number of mobile machine elements only creates small relative movements, such as for the stroke of the plunger, when high press forces are to be transmitted. The possibilities for situational or process-related forced releases are limited and it lacks an operating system for overload protection.
DE 25 07 098 describes a press which also requires a lot of enclosed space due to big constructional elements. The lever kinematics is disadvantageously disposed in parts in the sub-structure and in part in the upper support structure so that the upper support structure becomes an essential component of the press which absorbs forces. Integrating this press into the configuration of modern transfer presses or press lines is not possible without additional bypass routes, such as so-called block bypasses in the T-Track.
DD 119 014 describes a press where the construction height and complex guides do not allow for integration into lines of said transfer presses. The off-center forces described in the introduction are also poorly transferrable.
A punching press with a bottom drive is also described in EP 2 008 799 A1 in which the plunger is driven via tension columns by a drive mechanism with a crankshaft and a plunger disposed under the processing level. Bearing loads are hereby to be reduced by a special transmission mechanism and a distribution of the plunger forces and a high precision is to be achieved at high punching frequencies. With regard to the requirements for presses with a bottom drive that are to be developed, the disadvantage is here that work process-related settings can only be modified by adjusting the vertical position of the pivot point on the structure of the press. This solution does not allow for detecting and controlling the complex forces acting from the piston according to the processing requirements of the respective work piece across a major operating area. The distribution of the plunger forces influenced by the servo-motor could furthermore only be implemented in pure punching presses to a limited extent.
Presses were originally driven by an electric motor and an energy-storing flywheel. Energy efficient drives have meanwhile prevailed in the form of servo-electric drives. EP 1 880 837 A2 describes, for example, a press arrangement with energy management of a servo drive by which there is a sufficient capacity for absorbing additional energy on the one hand and enough energy is available at any given time in order to fulfill the respective press cycle.
In the context of an advantageous control and regulation of the movement of the plunger for servo-electrically driven presses, the problem of allowing for a position-controlled and force-controlled repeatable sequence of the movement of the plunger, wherein off-center forces are also to be controlled, is described in DE 10 2005 040 263 A1. In principle, this is solved by target torques of the servo-motors for driving the plunger being regulated depending on influencing variables by means of a position-curve slide controlled by a virtual drive shaft and a force and torque limiting value, which is controlled depending on the operating mode. The method and the device to this effect are supposed to be applicable to presses with a top drive and bottom drive, but in presses with a bottom drive, this solution requires particular, complex arrangements in the bottom drive and available space in the sub-structure, which is limited in this regard.
Considering the above conclusions, that:                presses with a sub-structure are to be implemented as large presses,        the line of action of off-center forces acting on the plunger always lies between the pivot points of the tie rods/connecting rods,        the tie rods/connecting rods in presses with a sub-structure are frequently guided in supporting stands, thus virtually forming a press frame for forces that may occur and are to be absorbed, because a supporting stand construction in said press frame is frequently chosen with regard to occurring forces according to the actions for processing the work pieces and to the reactions to press shocks or bending,        servo-electric drives must be implemented in the bottom drive, and        complex influences should not disturb the operation of the plunger,        
as a result of this technical overview (also with regard to DE 10 2005 040 263 A1) cost-effective solutions must be found for implementing and operating class-specific presses with bottom drives, also as large presses, without the disadvantages, such as, for example, a support stand construction, as indicated in the individual examples above.